1. Field of the Invention
The present invention relates to a technique of effecting an image processing on image information (image data) obtained by an image reader such as a scanner from an image recorded on a recording medium such as a photographic film. More particularly, the invention relates to a technique of effecting such processing on the image data for eliminating therefrom effect from a defect such as scratch or dust present on the recording medium.
2. Description of the Related Art
As printers for outputting to a print paper an image recorded on a photographic film such as a positive or a negative (to be referred to as “film” hereinafter), there are known an analog printer and a digital printer. The analog printer exposes the print paper with an optical beam transmitted through the film image. With the digital printer, the optical beam transmitted through the film image is subjected to a photoelectric conversion through a CCD (Charged Coupled Device) or the like and then digitized into image data and then the print paper is exposed with an optical beam modulated according to the image data.
With both types of printers above, if a defect such as scar or dust (to be generically referred to as “defect” hereinafter) is present on the film surface, this can result in such inconvenience as a unwanted local density variation or local deletion in the image printed on the print paper. For this reason, both types have employed a diffusion optical beam as the beam to be transmitted through the film.
In the case of the digital printer, the effect from a defect can be effectively eliminated by effecting an image processing on the digitized image data. Examples of such image processing method are disclosed in e.g. Japanese Patent Application “Kokai” No: Hei. 6-28468 (laid-open on Feb. 4, 1994, corresponding to U.S. Pat. No. 5,266,805), Japanese Patent Application “Kokai” No.: 2000-341473 (laid open on Dec. 8, 2000), and Japanese Patent Application “Kokai” No.: 2001-157003 (laid open on Jun. 8, 2001, corresponding to EP1100254A1).
With the image processing methods disclosed by the above publications, an infrared is transmitted through a film to obtain infrared image information. The infrared, when transmitted through the film, is diffusion by a defect on the film, but basically is not affected by pixels constituting the image of the film. Therefore, the infrared image information obtained from the infrared transmitted through the film will contain information of the defect only with high accuracy. Hence, from this infrared image information, it is possible to obtain the amount of the portion of the transmitted beam which was lost due to the presence of the defect. Then, by using this amount of loss due to the effect from the defect, the effect from the defect can be effectively eliminated from visible image information of the film.
In order to obtain such loss amount due to a defect, in the infrared image information, distinction needs to be made between a “non-defective” pixel (to be referred to a “non-deteriorated pixel” hereinafter) and a “defective” pixel (to be referred to as “deteriorated pixel” hereinafter). A non-defective portion of the film allows transmission of a greater amount of the beam than a defective portion, so that the non-deteriorated pixel in the infrared image information has a greater pixel value (density value) than the deteriorated pixel in the same. Therefore, the distinction between the non-deteriorated pixel and the deteriorated pixel is possible by setting a threshold value for pixel values of an infrared image information and then judging a pixel having a pixel value higher than the threshold value as a non-deteriorated pixel while judging a pixel having a pixel value smaller than the threshold value as a deteriorated pixel.
However, as films have differing transmission amounts for an infrared depending on the film type, the maker, sensitivity, etc., it is not possible to make the above distinction with a single threshold value. As a solution to this problem, the above-cited Japanese Patent Application “Kokai” No.: 2000-341473 discloses a method in which film characteristics information relating to e.g. positive/negative, presence/absence of a magnetic layer is obtained for each particular film and then the threshold value is adjusted based on this information.
Incidentally, if a pixel value of a non-deteriorated pixel (to be referred to as “non-deteriorated pixel value” hereinafter) can be obtained from the infrared image information for each film, the above problem can be solved by setting this non-deteriorated pixel value as the threshold.
Ideally, the non-deteriorated pixel value can be considered as a maximum value of pixel values of the infrared image information. As a matter of fact, even such non-deteriorated pixel value can vary, due to noise, sensitivity of each image pickup element of a CCD camera, density irregularity in the film, etc. Namely, it will be problematic to simply interpret a maximum pixel value as the non-deteriorated pixel value for its use as the threshold value.
On the other hand, as a value approximating the non-deteriorated pixel value, an average value of pixel values of the infrared image information (to be referred to as an “average pixel value” hereinafter) can be cited for example. Yet, since deteriorated pixels have smaller pixel values than non-deteriorated pixels as described hereinbefore, such average pixel value will be lower if a large amount of defect is present on the film, thus resulting in a significant difference between the non-deteriorated pixel value and the average pixel value.
This phenomenon will be described in greater details with reference to FIGS. 8 and 9. FIGS. 8A and 8B show infrared image information (image data) obtained from a film having a small amount of defect and frequency distribution of pixel values of this infrared image information, respectively. Similarly, FIGS. 9A and 9B show infrared image information (image data) obtained from a film having a large amount of defect and frequency distribution of pixel values of this infrared image information, respectively.
In the case of the film having a small amount of defect as shown in FIG. 8A, there is observed a conspicuous concentration of the frequency distribution of the pixel values of the infrared image information obtained therefrom in the vicinity of the non-deteriorated pixel value: CF as shown in FIG. 8B. In this case, though not shown, the average pixel value: Mean [IR] of the infrared image is approximate to the non-deteriorated image value: CF.
On the other hand, in the case of the film having a large amount of defect as shown in FIG. 9A, the frequency distribution of the pixel values of the infrared image information obtained therefrom is shifted from the non-deteriorated pixel value: CF to the lower pixel value side (to the left in the graph). In this case, the average pixel value: Mean [IR] (denoted as: Ave in the graph of FIG. 9B) of the infrared image information is smaller than the non-deteriorated pixel value: CF. Then, if the distinction of deteriorated pixels is made with using this average value: Mean [IR] as the threshold value, those pixels having pixel values between the average pixel value: Mean [IR] and the non-deteriorated pixel value: CF will not be judged as deteriorated pixels although they are really so.